Polyester core materials and structural sandwich composites thereof

ABSTRACT

High-strength, chemically and thermally stable, closed-cell foams, useful as structural core materials in sandwich composites. The core materials of the invention display anisotropic properties. The core materials of the invention are amenable to vacuum-mediated resin bonding to composite skins to provide lightweight, high-strength structural sandwiches suitable for use in a variety of applications, such as marine applications, construction, aviation, rapid transit, and recreational vehicles.

[0001] This is a division of application Ser. No. 10/183,841, filed Jun.27, 2002, which application is hereby incorporated herein by referencein its entirety.

1. FIELD OF THE INVENTION

[0002] The invention is directed to chemically and thermally-stablestructural core materials comprising compressed foamed polyester strandsand methods for their preparation. The invention is also directed tostructural sandwich composites constructed from such core materials andmethods for their preparation.

2. BACKGROUND OF THE INVENTION

[0003] Structural sandwich composites—which are sandwich-likearrangements of a relatively low-density core material bonded betweencomparatively thin, high-strength and high-stiffness skins—are used in awide variety of applications that require lightweight, yet structurallystrong materials. To name but a few applications, structural sandwichcomposites are used in boating, construction, aviation, rapid transit,and recreational vehicles. Structural sandwich composites are usefulbecause of their high strength and low weight per unit area. When bondedbetween skins, the low-density core provides a large strength andstiffness enhancement over the skins alone, but adds only acomparatively small weight. To illustrate the benefits of structuralsandwich composite construction, consider that dividing a material(e.g., aluminum or fiberglass) into two skins and bonding a corematerial that is twice the original material's thickness in betweenthem, results in a composite having a stiffness 7 times greater and astrength 3.5 times greater than the original material's while having adensity only 1.03 times that of the original material. ANDREW C.MARSHALL, COMPOSITE BASICS 3-1 (5th ed. 1998).

[0004] How well a sandwiched core material functions in real-worldapplications can be predicted from laboratory measurements of itscompression strength and modulus, tensile strength and modulus, andshear strength and modulus.

[0005] The properties of the core material are of great importance.Desirable properties include high strength, low density, rigidity, highchemical and heat resistance, and low cost. The most common corematerials are wood, honeycomb structures, and foams comprising boththermoplastic and thermosetting compositions. Wood core materials sufferfrom variations in properties and are susceptible to fungal decay,especially in marine use. Honeycomb cores are of an open structure,i.e., comprised of contiguous, connected, and/or interlocked cells, andare typically constructed from rigid materials, such as thermoplastics,fiberglass, aluminum, and stainless steel. While honeycomb-corematerials provide strong, high-quality, chemically resistant composites,they are difficult to manufacture. The connected nature of the cellsprecludes composite manufacture by vacuum-mediated resin techniquesbecause the vacuum draws the resin into the individual cells.Furthermore, honeycomb cores are not suitable for marine applicationsbecause a crack in the composite skin can lead to the entire compositefilling with water. Closed-cell thermoplastic or thermosetting foamsavoid some of these problems, but generally are thermally and chemicallysensitive; thus, their composites cannot be used in certainhigher-temperature applications. A further disadvantage ofthermoplastic- or thermosetting-foam core materials is that certainresin-type adhesives can significantly degrade them, both chemically andvia the heat evolved during the cure process.

[0006] Skins can be attached to core materials by a variety of methods.One of the most popular methods, because of the high shear strength ofthe resulting composite, is bonding the skins to the core with a resin(the resin-cure method). The resin-cure method provides structuralsandwich composites with excellent skin-core adhesion and delaminationresistance. In the resin-cure method, an uncured resin is applied to thecontacting surfaces, the core and the skins are contacted, and bondingresults upon resin cure. Often, a reinforcing material such as aglass-fiber fabric or mat is combined with the uncured resin to improvestrength and stiffness in the resulting joint. During resin cure,substantial heat is generated.

[0007] Vacuum-bagging and vacuum-injection-molding techniques are usedcommercially to introduce the resin between the skins and suitable coressee, for example, U.S. Pat. No. 6,159,414 (issued May 18, 1999); U.S.Pat. No. 5,316,462 (issued May 31, 1994); and U.S. Pat. No. 5,834,082(issued Nov. 10, 1998). In this process, vacuum is used to draw theuncured resin between the core and skin. Advantageously, the vacuumremoves resin fumes as well as shields the uncured resin from air.

[0008] With some core materials, however, such as honeycomb structures,vacuum-mediated resin application is difficult or impossible. Andunfortunately, in these cases, open-air resin application is proscribedbecause the hazardous resin fumes are not contained and resin curing canbe inhibited by air and moisture. Thus, thermoplastic or thermosettingfoams are ideal in that they do not suffer from the biodegradability ofwood cores and are amenable to vacuum-mediated resin application. But aserious drawback with thermoplastic- or thermosetting- foam cores isthat the heat evolved during resin cure and the chemically corrosiveproperties of the resin can degrade them, resulting in weakercomposites.

[0009] Thermoplastic polyester resins, such as polyethyleneterephthalate (PET) and polybutylene terephthalate (PBT) that have beenpre-treated with branching agents (hereinafter “branched polyesters”)yield closed-cell foams having excellent strength and mechanicalproperties, low density, and high chemical and thermal resistance. Thebranching agents, which have multiple chemical-reaction sites, functionby chemically condensing two or more polyester chains (“branching”).This branching gives the pre-foam polyester melt viscoelastic propertiesmore suitable for foaming, leading to higher quality foams. Polyesterfoams, prepared from branched polyesters, such as branched polyethyleneterephthalate, have been disclosed in U.S. Pat. No. 5,000,991 (issuedMar. 19, 1991); U.S. Pat. No. 5,229,432 (issued Jul. 20 1993); U.S. Pat.No. 5,340,846 (issued Aug. 23, 1994); U.S. Pat No. 5,362,763 (issuedNov. 8, 1994); U.S. Pat. No. 5,422,381 (issued Jun. 6, 1995); U.S. PatNo. 5,679,295 (Oct. 21, 1997); U.S. Pat. No. 5,681,865 (Oct. 28, 1997);U.S. Pat. No. 6,342,173 (issued Jan. 29, 2002), each of which eightpatents are hereby incorporated by reference herein. These foams areclosed-cell structures with low densities, excellent mechanicalproperties, and high thermal and chemical resistance. Regrettably,because the process used for their manufacture leads to irregularsurfaces, such foams make mediocre to poor core materials. The irregularsurfaces promote weak bonding to the composite skin and wide cell-sizedistribution and, therefore, poor mechanical properties. To explain morefully, polyesters are generally foamed by extruding a pressurizedmixture of a branched-polyester melt and a volatile, organic expandingor “blowing agent” through an annular or slit die. Upon entering ambientpressure, the blowing agent evaporates and the polyester foams. Thisprocess suffers in that if the die opening size surpasses a criticallimit, extruder pressure cannot be maintained. Furthermore, as the dieopening is enlarged to the size required for use as a core material,blowing-agent evaporation throughout the material becomes non-uniformleading to erratic cell-size distribution, oversized cells, and anirregular surface.

[0010] Coalesced-strand polyester foams are more suitable as corematerials because they can be produced in thicker size with a uniformdistribution of small cells. Coalesced-strand polyester foams aredisclosed in U.S. Pat. No. 5,475,037 (issued Dec. 12, 1995). Generally,coalesced-strand thermoplastic foams are prepared by melting athermoplastic resin, mixing the melt with a blowing agent, and extrudingthe resulting gel through a multi-orifice die. The orifices are soarranged such that some contact between adjacent strands occurs duringfoaming, and the contacting strand surfaces adhere to one anotherresulting in a coalesced-strand structure. These strand foams, however,are not used as core materials. Tenacious, tough thermoplastic resinssuch as polypropylene or polyethylene which generally exhibit lowerstiffness, may be advantageously used for some applications, such asshock absorbers (see, e.g., U.S. Pat. No. 6,213,540 (issued Apr. 10,2001)) but they offer poor performance as composite core materials forwhich high strength and stiffness are desirable.

[0011] In view of the above, there is a need for low-density closed-cellcore materials that are rigid, strong, chemically and thermallyresistant, and amenable to vacuum-mediated resin application.

3. SUMMARY OF THE INVENTION

[0012] The invention provides high-strength, chemically and thermallystable, closed-cell foams, useful as core materials in composites. Thecore materials of the invention comprise multiple foamed polyesterstrands, compressed together (or shaped) to form a unitary, closed cellfoam displaying anisotropic properties.

[0013] A further feature of core materials of the invention is that,although they are manufactured by extruding through a multi-orifice die,there are substantially no voids in between the strands (no inter-strandvoids). This is a result of the special shaping process and shapingapparatus described in more detail herein. The shaping process can beadjusted to completely remove the strand appearance of the corematerial. That is, if the core material is cut perpendicular to thestrand direction, strands are no longer visible to the human eye. Thestrands have coalesced to a degree wherein the core material appears tobe completely unitary. Thus, to the human eye, the core materials of theinvention appear identical to a conventional foam board manufactured byextruding through a single-orifice, standard slot die. However, the corematerials of the invention display improved properties over conventionalslot-die produced foam boards. And, in contrast to such conventionalfoam boards, the core materials of the invention are significantlyanisotropic in character and have an unusual cell size distribution. Thecell-size distribution of core materials of the invention defines aplurality of “discrete volumes”, running parallel to the stranddirection. These “discrete volumes” comprise an interior section runningparallel to the strand direction and a “jacket” surrounding the“interior section of the discrete volume”. The “interior section of thediscrete volume” has closed cells of average cell size relatively largerthan the average cell size of the cells in the surrounding “jacket”. Inother words, the average-cell diameter is smaller where the strands haveintersected and merged than at the original strand's interior. This canbe described as a pseudo honeycomb structure.

[0014] Because of their high thermal and chemical resistance, the corematerials of the invention are amenable to vacuum-mediated resin bondingto composite skins to provide lightweight, high-strength, buoyant, andwatertight structural sandwich composites suitable for use in a varietyof applications, such as construction, boats, ships, and other marineapplications, aviation, rapid transit, and recreational vehicles. Infact, the core materials of the invention are compatible with just aboutall resins, and can accept high molding temperatures and pressures. Theycan be processed with nearly all composite fabrication techniquesincluding contact molding, vacuum bagging, resin infusion, autoclave,RTM, match metal molding, pre-preg and others.

[0015] In one embodiment, the invention is directed to a method formaking a composite comprising: extruding a foamable gel comprising ablowing agent and a foamable polyester through a multi-orifice die togive a plurality of strands; foaming the strands to form amulti-stranded foamed article; shaping the multi-stranded foamed articleto give a core material; and bonding the core material to one or morestructural skins.

[0016] In another embodiment, the invention is directed to a compositecomprising a foamed polyester core material bonded to one or morestructural skins, wherein the core material comprises a plurality ofdiscrete volumes, each discrete volume comprising an interior sectionand a corresponding jacket, wherein the average cell size in theinterior section is larger than the average cell size in the jacket.

[0017] In still another embodiment, the invention relates to a compositecomprising a foamed polyester core material bonded to one or morestructural skins, wherein the core material comprises a plurality offoamed strands, wherein there are substantially no inter-strand voids.

[0018] In yet one more embodiment, the invention is directed to acomposite comprising a foamed polyester core material bonded to one ormore structural skins, wherein the core material is anisotropic.

[0019] In another embodiment, the invention relates to a core materialcomprising foamed polyester, wherein the foamed polyester comprises aplurality of discrete volumes, each discrete volume comprising aninterior section and a corresponding jacket, wherein the average cellsize in the interior section is larger than the average cell size in thejacket. Preferably, the core material is anisotropic.

4. BRIEF DESCRIPTION OF THE FIGURES

[0020] These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings where:

[0021]FIG. 1 is a flowchart outlining the general steps that may be usedto extrude a foamable polyester gel to obtain a core material of theinvention;

[0022]FIG. 2 is a drawing representing a unitary multi-stranded foamedcore material of the invention prior to conversion into a core materialof the invention by compression shaping to remove the voids between thestrands;

[0023]FIGS. 3A to 3C are perspective, side, and front view drawingsrespectively of a shaper of the invention;

[0024]FIG. 4 is a drawing of a cross-sectional view of a core materialof the invention;

[0025]FIGS. 5, 6, and 7 are graph plots of shear strength versus density(FIG. 5); shear modulus versus density (FIG. 6); and shear elongation atbreak versus density (FIG. 7) conducted in a direction end-strand,transverse, and longitudinal to the strand direction respectively of PETcore materials of the invention;

[0026]FIGS. 8 and 9 are graph plots of tensile strength versus density(FIG. 8) and tensile modulus versus density (FIG. 9) of PET corematerials of the invention conducted in a direction end-strand andtransverse to the strand direction respectively;

[0027]FIGS. 10 and 11 are graph plots of compression strength versusdensity (FIG. 10) and compression modulus versus density (FIG. 11) ofPET core materials of the invention conducted in a direction end-strandand transverse to the strand direction respectively; and

[0028]FIGS. 12, 13, and 14 are graph plots comparing the properties ofshear strength; shear modulus; and shear elongation at break,respectively, of PET core materials of the invention versusconventional, slot-die extruded PET foam boards over a density range.

5. DETAILED DESCRIPTION OF THE INVENTION

[0029] The core materials of the invention may be prepared as follows. Afoamable polyester, of suitable melt rheology, is heated above its meltpoint to form a polyester melt. The heated mixture is pressurized in anextruder and a blowing agent is blended into the melt to form a foamablegel. During blowing-agent addition and mixing, the pressure ismaintained above the blowing agent's equilibrium vapor pressure at theoperating temperature of the foamable gel. The gel is cooled andextruded through a multi-orifice die of desired design. Upon enteringambient pressure, the blowing agent boils and expands thereby foamingthe strands. As the strands foam, they coalesce resulting inmulti-stranded polyester foam. At this point, the foam has voids orchannels running parallel to the strands. The voids are removed bycompressing the foam in a special shaping process. Temperatures,pressures, and extrusion rates will depend upon the specific polyester,additives, blowing agents, equipment, die design, and on the propertiesdesired in the final foam product. The core materials of the inventionmay be bonded to skins to form composites by well-known methods in theart, such as vacuum-mediated resin application.

5.1 Definitions

[0030] 5.1.1 Structural Sandwich Composite

[0031] As used herein, the phrase “structural sandwich composite” meansan article comprising a core material integrally bonded to one or morestructural skins. Preferably a “structural sandwich composite” means asandwich-like article comprising a core material integrally bondedbetween structural skins. The phrase “Integrally bonded” means that theskin is bonded to the core material substantially throughout the skin'sentire contact area. A “structural sandwich composite of the invention”means a structural sandwich composite comprising a core material of theinvention. Examples of suitable structural skins include, but are notlimited to, thermoplastic polymers and thermosetting polymers,optionally reinforced with glass fibers, metallic fibers, inorganicfibers, or carbon fibers; wood; inorganic materials, such as fiberglass;and metallic materials, such as aluminum and stainless steel and manyothers, which are well known to those of skill in the art.

[0032] 5.1.2 Core Materials of the Invention

[0033] As used herein, the phrase “core materials of the invention”means a foamed article prepared by extruding a melt comprising afoamable polyester and one or more blowing agents through amulti-orifice die, according to the methods described more fully herein,to give a multi-stranded foamed core material. Preferably, the coalescedmulti-stranded foamed core material is further compressed (via shaping)to remove substantially all the voids between the individual strands.Thus, the multi-stranded foamed core material is transformed via shapinginto a unitary closed-cell structure, wherein certain foam-cell walls(those corresponding to the surface where the exterior of the strandswere compressed together during shaping) are of increased (generallydouble) thickness over those of the strand interior. The strand densityof the multi-stranded foamed core material prior to shaping will governthe relative number of cells having such increased thickness. Such astructure is referred to herein as a “compressed-strand structure” givesthe core materials of the invention advantageous structural andmechanical properties over traditional foam core materials.

[0034] 5.1.3 Pre-Shaped and Shaped Core Materials of the Invention

[0035] As used herein, the phrase “pre-shaped core material” means a“core material of the invention” prior to its being shaped by a shaperof the invention, which shaper and shaping process is more fullydescribed in Section 5.3.2 below. In some cases, pre-shaped corematerials of the invention may have gaps or air pockets between thestrands. The shaping process, which results in a “shaped core materialof the invention”, compresses the strands thereby removing the gaps orair pockets. Preferably, core materials of the invention are shaped.

[0036] 5.1.4 Strand Direction

[0037] As used herein, the phrase “strand direction” means the axisalong which a core material of the invention was extruded duringproduction. This is an important reference point since, in some cases,the strand character of core materials of the invention is not visibleto strand compression and merger during the special shaping disclosedherein (see Section 5.3.2 below). The “strand direction” influences theanisotropic physical properties of core materials of the invention. Forexample, the core materials of the invention exhibit greater compressivestrength in the “strand direction”. This is discussed more fully inSection 5.4.2 below.

[0038] 5.1.5 End Strand Direction in Composites of the Invention

[0039] The phrase “end-strand direction” is used in reference tocomposites of the invention wherein the composite's skins are bonded toa core material of the invention perpendicular to the strand direction.In such circumstances, the “end-strand direction” means a direction oraxis perpendicular to the strand direction (therefore, also parallel tothe composite skins).

[0040] 5.1.6 Longitudinal Direction in Composites of the Invention

[0041] The phrase “longitudinal direction” is used in reference tocomposites of the invention wherein the composite's skins are bonded toa core material of the invention such that the skins are parallel to thestrand direction. In such circumstances, the “longitudinal direction”means a direction or axis parallel to the strand direction.

[0042] 5.1.7 Transverse Direction in Composites of the Invention

[0043] The phrase “transverse direction” is used in reference tocomposites of the invention wherein the composite's skins are bonded toa core material of the invention such that the skins are parallel to thestrand direction. In such circumstances, the “transverse direction”means a direction or axis perpendicular to the strand direction.

[0044] 5.1.8 Foamable Polyester

[0045] As used herein, the phrases “foamable polyester” or “foamablepolyester resin” mean any thermoplastic or thermoplastic mixturecomprising polyester, a branched polyester, a polyester co-polymer, or abranch polyester co-polymer that can be effectively foamed to yield acore material of the invention.

[0046] 5.1.9 Polyester Co-Polymer

[0047] As used herein, the phrase “polyester co-polymer” means apolyester prepared, according to well-known methods, by co-polymerizingan ester monomer and one or more other monomers.

[0048] 5.1.10 Branched Polyester

[0049] As used herein, the phrase “branched polyester” means a polyesteror a polyester co-polymer that has been condensed with one or morebranching agents. Preferably, branched polyesters are foamable.

5.2 Foamable Polyesters for use in the Invention

[0050] Although polyesters have excellent chemical and thermal stabilityand structural properties, foaming is often difficult due to their meltrheology (e.g., low melt strength and low melt viscosity). In somecases, this can be overcome by using special processing equipment. Butthe more common way to improve polyester foaming characteristics is toalter the melt rheology by pre-treating them with branching agents, forexample, polyfunctional carboxylic acids, polyfunctional anhydrides, andpolyhydroxyl compounds according to well-known methods.

[0051] Preferably, foamable polyester resins in the invention have acrystallinity of from about 5% to about 100%, more preferably, of fromabout 10% to about 60%, most preferably, of from about 25% to about 45%,and even more preferably, of from about 28% to about 39%, as measured bydifferential scanning calorimetry.

[0052] Preferably, foamable polyester resins have the following meltrheology and properties: (1) a melt strength of from about 1 to about 60centinewtons, preferably, of from about 5 to about 30 centinewtons; (2)a melt viscosity of from about 30,000 to about 500,000 poises,preferably, of from about 200,000 to about 300,000; and (3) an inherentviscosity of from about 0.5 to about 1.95 dl/g, preferably, of fromabout 0.7 to about 1.2 dl/g.

[0053] 5.2.1 Crystallinity

[0054] The crystallinity of foamable polyester resins for use in theinvention may be measured by differential scanning calorimetry. Thisinvolves absorbed-energy measurement as a polyester-resin sample isheated to and beyond its crystalline melting point. The crystallinity isdefined as the ratio of the energy absorbed per unit mass to thetheoretical heat of fusion. For polyethylene terephthalate resin, thetheoretical heat of fusion is 26.9 kJ/g-mole of repeat units, or moreconveniently 140 Joules per gram of resin. 3 B. WUNDERLICH , CrystalMelting, in MACROMOLECULAR PHYSICS (1980).

[0055] As is well known in the art, a differential scanning calorimetercan measure very small quantities of energy that are absorbed orreleased from a sample material. In a typical procedure, a 5 mg to 10 mgpolyester resin sample is sealed in an aluminum pan and placed in thecalorimeter's sample cell. Typically, the sample is purged with an inertgas, such as nitrogen to preclude sample oxidation. Then, the sampletemperature is ramped, for example, 10° C. per minute while thedifferential scanning calorimeter measures the energy absorbed orreleased. The enthalpy of melting or heat of fusion is determinedautomatically by the differential scanning calorimeter with softwarethat integrates the area under the melting peak. For quality assurance,the foamable polyester's crystallinity should be measured bydifferential scanning calorimetry prior to foaming. For referencesrelating to differential scanning calorimetry, see Dole et al., 20 J.CHEM. PHYSICS 781 (1952); Wunderlich et al., 24 J. POLYMER SCI. 201(1957); Quinn et al., 80 J. AM. CHEM. SOC. 3178 (1958); Wunderlich etal., Part 2A J. POLYMER SCI. 987 (1967); Atkinson et al., 65 TRANS.FARADAY SOC. 1764 (1969); Richardson, Part C J. POLYMER SCI. 251 (1972).

[0056] 5.2.2 Melt Strength

[0057] The melt strength may be measured according to the procedure setforth in American Society for Testing and Materials (“ASTM”) D3835“Standard Test Method for Determination of Properties of PolymericMaterials by Means of a Capillary Rheometer” measured at 280° C. Thistest method describes measurement of the rheologic properties ofpolymeric materials at various temperatures and shear rates common toprocessing equipment. It covers measurement of melt viscosity,sensitivity, or stability of melt viscosity with respect to temperatureand polymer dwell time in the rheometer, die swell ratio (polymermemory), and shear sensitivity when extruding under constant rate orstress. The techniques described permit the characterization ofmaterials that exhibit both stable and unstable melt viscosityproperties. Other suitable methods are set forth in U.S. Pat. No.5,362,763 (issued Nov. 8, 1994); U.S. Pat. No. 6,350,822 (issued Feb.26, 2002); or U.S. Pat. No. 6,251,319 (issued Jun. 26, 2001) whichpatents are hereby incorporated by reference herein using, a RheotensMelt Strength Tester, Type 010.1, supplied by GottfertWerkstoff-Prufinaschinen Gmbh of Buchen, Germany. This test involvesdrawing an extruded strand of polymer vertically into the nip betweentwo counter-rotating nip rollers using piston rate of 0.2 mm/sec. Thestrand is extended using a Brabender Plasticorder single screw extruderof screw diameter 19 mm and length to diameter ratio (L/D) of 25. Theextruded material exits through a right angle capillary die of length 30mm and 1 mm diameter at a rate of 0.030 cc/sec to a length of 41.9 mm at270° C. The strand is then stretched at a constant acceleration whilemeasuring the elongation. The temperature profile used was uniform alongthe length of the barrel of the extruder and the die and was set at 280°C. The nip rollers are mounted on a balance arm, which allows the forcein the drawing strand to be measured. The velocity of the nip rolls isincreased at a uniform acceleration rate. As the test proceeds, theforce increases until eventually the strand breaks. The force atbreakage is termed the “melt strength”.

[0058] 5.2.3 Melt Viscosity

[0059] The melt viscosity may be measured according to the procedure setforth in ASTM D4440-01 “Standard Test Methodfor Plastics: DynamicMechanical Properties: Melt Rheology”, hereby incorporated by referenceherein. ASTM test procedures are compiled, reviewed, and published bythe American Society for Testing and Materials, which is a voluntarystandards development organization operating out of West Conshohocken,Pa. This test method describes the use of dynamic mechanicalinstrumentation for gathering and reporting the rheologic properties ofthermoplastic resins. It may be used as a test method for determiningthe complex viscosity and significant viscoelastic characteristics ofpolyester thermoplastics as a function of frequency, strain amplitude,temperature, and time. Rheometric calculations from data obtained byASTM D4440-01 may be performed according to ASTM D4065-01 “StandardPracticefor Plastics: Dynamic Mechanical Properties: Determination andReport of Procedures”, hereby incorporated by reference herein. Thisreport recites laboratory practice for determining dynamic mechanicalproperties of specimens subjected to various oscillatory deformations ona variety of instruments of the type commonly called dynamic mechanicalanalyzers or dynamic thermomechanical analyzers.

[0060] 5.2.4 Inherent Viscosity

[0061] The inherent viscosity may be measured according to ASTM TestMethod D4603-96 “Standard Test Method for Determining Inherent Viscosityof Poly(Ethylene Terephthalate) (PET) by Glass Capillary Viscometer”,hereby incorporated by reference herein (see e.g., U.S. Pat. No.5,422,381, hereby incorporated by reference herein). This test method isfor the determination of the inherent viscosity of polyethyleneterephthalate soluble at 0.50% concentration in a 60/40phenol/1,1,2,2-tetrachloroethane solution by means of a glass capillaryviscometer. Some highly crystalline forms of polyethylene terephthalatemay not be soluble in this solvent mixture and will require a differentprocedure The inherent viscosity values obtained by this test method arecomparable with those obtained using differential viscometry describedin ASTM Test Method D5225-98 “Standard Test Method for MeasuringSolution Viscosity of polymers with a Differential Viscometer”, herebyincorporated by reference herein.

[0062] 5.2.5 Polyesters

[0063] Polyesters that are foamable or can be converted to foamablepolyesters are suitable for use in the invention. Preferably, thepolyester is polyethylene terephthalate (PET), polyethylene naphthalate(PEN), or polybutylene terephthalate (PBT), more preferably,polyethylene terephthalate. The polymer's molecular weight is notcritical, generally molecular weights of 100,000 D to 1,000,000 D aresuitable depending on the polymer type.

[0064] Preferred polyesters include, but are not limited to, foamablepolyesters derived from reaction of aromatic dicarboxylic acid and adihydric alcohol, such as those polyesters described in U.S. Pat. No.5,110,844 (issued May 5, 1992), hereby incorporated by reference herein.Specific examples of preferred foamable polyesters include, but are notlimited to, foamable polyethylene terephthalate (PET), foamablepolybutylene terephthalate (PBT), foamable polyethylene naphthalate(PEN), foamable copolymers of PET, foamable copolymers of PBT, foamablecopolymers of PEN, foamable liquid-crystalline polyesters.

[0065] Foamable blends of polyesters and other thermoplastics are alsosuitable for use in the invention. Suitable blends include foamablepolyester/polycarbonate blends as described in U.S. Pat. No. 4,833,174(issued May 23, 1989) and U.S. Pat. No. 4,462,947 (issued Jul. 31,1984), both of which are hereby incorporated herein by reference.Suitable blends also include foamable polyester/polyolefin blends asdescribed in U.S. Pat. No. 4,981,631 (issued Jan. 1, 1991) and U.S. Pat.No. 5,128,202 (issued Jul. 7, 1992), both of which are herebyincorporated by reference herein. Suitable polyolefins include, but arenot limited to, those listed in column 4, line 33 through column 4, line35 of U.S. Pat. No. 4,981,631, which disclosure is hereby incorporatedby reference herein.

[0066] Foamable co-polymers of polyesters and other thermoplastics arealso suitable for use in the invention. Suitable polyester co-polymersinclude, but are not limited to, those listed in column 3, line 3through column 4, line 41 of U.S. Pat. No. 5,475,037 (issued Dec. 121995), which disclosure is hereby incorporated by reference herein andpoly-2-hydroxy-6-naphthoic acid and polynaphthalene terephthalate, whichis a copolymer of 2,6-dihydroxynaphthalene and terephthalic acid.

[0067] 5.2.6 Sources of Foamable Polyesters

[0068] Polyesters for use in the invention are readily availablecommercially or can be synthesized by well-known literature methods, forexample see, GEORGE ODIAN, PRINCIPLES OF POLYMERIZATION 97-100 (3d ed.1991); FRED W. BILLMEYER, JR., TEXTBOOK OF POLYMER SCIENCE 63, 149,225-227, 434, 452-454(2d ed., 1971), both of which are herebyincorporated herein by reference. In addition, foamable polyethyleneterephthalate and other foamable polyesters are available commercially,for example, from Mossi & Ghisolfi Polymers (Houston, Tex.) and Du Pont(Wilmington, Del.).

[0069] Suitable procedures for preparing foamable polyesters bycondensing polyesters with branching agents are disclosed in U.S. Pat.No. 3,553,157 (issued Jan. 5, 1971), U.S. Pat. No. 4,132,707 (issued Jan2, 1979); U.S. Pat. No. 4,145,466 (issued Mar. 20, 1979); U.S. Pat.4,462,947 (issued Jul. 31, 1984); U.S. Pat. No. 4,999,388 (issued Mar.12, 1991); U.S. Pat. No. 5,000,991 (issued Mar. 19, 1991); U.S. Pat. No.5,110,844 (issued May 5, 1992); U.S. Pat. No. 5,128,383 (issued Jul. 7,1992); U.S. Pat. No. 5,134,028 (issued Jul. 28, 1992); U.S. Pat. No.5,288,764 (issued Feb. 22, 1994); U.S. Pat. No. 5,362,763 (issued Nov.8, 1994); U.S. Pat. No. 5,422,381 (issued Jun. 6, 1995); U.S. Pat. No.5,482,977 (issued Jan, 9, 1996); U.S. Pat. No. 5,696,176 (issued Dec. 9,1997); U.S. Pat. No. 5,229,432 (issued Jul. 20, 1993); and U.S. Pat. No.6,350,822 (issued Feb. 26, 2002), all of which are hereby incorporatedby reference herein.

[0070] Suitable polyfunctional carboxylic acid branching agents havethree or more carboxylic acid functions per molecule and include, butare not limited to, trimesic acid; pyromellitic acid;benzophenonetetracarboxylic acid; 2,3,6,7-napthalenetetracarboxylicacid; 1,2,5,6-napthalenetetracarboxylic acid;1,2,3,4-cyclobutanetetracarboxylic acid;tetrahydrofuran-2,3,4,5-tetracarboxylic acid;2,2-bis(3,4-dicarboxyphenyl)propane; and3,3′,4,4′-biphenyltetracarboxylic acid. Such branching agents arecommercially available or prepared by well-known methods.

[0071] Suitable polyhydroxyl compounds and equivalents (i.e., a compoundthat can be converted in situ to a polyhydroxy alcohol, e.g., esters ofpolyhydroxyl compounds) have at least three hydroxyl groups orhydroxyl-group equivalents per molecule and include, but are not limitedto, glycerol; trimethylolpropane; trimethylolethane; pentaerythritol,dipentaerythritol, tripentaerythritol, and esters thereof;1,2,6-hexanetriol; sorbitol; glycerol tripropylate; glyceroltribenzoate; 1,1,4,4-tetrakis(hydroxymethyl)cyclohexane;tris(2-hydroxyethyl)isocyanurate; ethylene oxide; and propylene oxide.Polymers or copolymers having polyhydroxyl groups are also suitable andinclude, but are not limited to, poly(ethylene-co-vinyl alcohol) andpoly(ethylene-co-vinyl acetate). Preferably, the polyhydroxyl compoundis pentaerythritol, dipentaerythritol, tripentaerythritol, or an esterthereof; trimethylolpropane; trimethylolethane; glycerol; or any mixturethereof. Such branching agents are commercially available or prepared bywell-known methods.

[0072] Polyfunctional acid anhydride branching agents are the mostpreferred class of branching agents for preparing suitable foamablepolyesters. Suitable polyfunctional acid anhydride branching agents haveat least one anhydride group and one or more additional carboxylic,hydroxyl, or anhydride groups per molecule and include, but not limitedto, pyromellitic dianhydride; 1,2,3,4-cyclobutanetetracarboxylic aciddianhydride; benzophenonetetracarboxylic acid dianhydride;diphenylsulfone tetracarboxylic dianhydride;bis(3,4-dicarboxyphenyl)ether dianhydride;bis(3,4-dicarboxyphenyl)thioether dianhydride; bisphenol-A bisetherdianhydride; 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride;2,3,6,7-napthalenetetracarboxylic acid dianhydride;bis(3,4-dicarboxyphenyl)sulfone dianhydride;1,2,5,6-napthalenetetracarboxylic acid dianhydride;2,2′,3,3′-biphenyltetracarboxylic acid dianhydride; hydroquinonebisether dianhydride; 3,4,9,10-perylene tetracarboxylic aciddianhydride; tetrahydrofuran-2,3,4,5-tetracarboxylic acid dianhydride;3,3′,4,4′-biphenyltetracarboxylic acid dianhydride; and4,4′-oxydiphthalic dianhydride. Polymers or copolymers containing anacid anhydride component are also suitable. Preferred polyfunctionalacid anhydride branching agents are pyromellitic dianhydride;1,2,3,4-cyclopentanetetracarboxylic acid dianhydride;1,2,3,4-cyclobutanetetracarboxylic acid dianhydride; andtetrahydrofuran-2,3,4,5-tetracarboxylic acid dianhydride, morepreferably, pyromellitic dianhydride. The preferred polyfunctional acidanhydride branching agent is pyromellitic dianhydride, which iscommercially available, for example, from Aldrich.

[0073] Mixtures of polyfunctional carboxylic acids, polyhydroxylcompounds; and/or polyfunctional acid anhydrides, are also suitablebranching agents, for example, a mixture comprising a polyhydroxylalcohol and a polyfunctional acid anhydride.

5.3 Preparation of Core Materials of the Invention

[0074] Well-known methods can be used in the initial, multi-strandedfoaming process to produce coalesced multi-stranded foamed articles,which are precursors to the unitary core materials of the invention.Such procedures are described in detail in U.S. Pat. No. 3,573,152(issued Mar. 30, 1971); U.S. Pat. No. 4,122,047 (issued Oct. 24, 1978);U.S. Pat. No. 4,462,947 (issued Jul. 31 1984); U.S. Pat. No. 4,824,720(Apr. 25, 1989); U.S. Pat. No. 4,833,174 (issued May 23, 1989); U.S.Pat. No. 4,981,631 (Jan. 1, 1991); U.S. Pat. No. 5,246,976 (issued Sep.21, 1993); U.S. Pat. No. 5,254,400 (issued Oct. 19, 1993); U.S. Pat. No.5,340,846 (issued Aug. 23, 1994); U.S. Pat. No. 5,360,829 (Nov. 1,1994); U.S. Pat. No. 5,362,763 (issued Nov. 8, 1994); U.S. Pat. No.5,391,582 (issued Feb. 21, 1995); U.S. Pat. No. 5,399,595 (Mar 21,1995); U.S. Pat. No. 5,422,381 (issued Jun. 6, 1995); U.S. Pat. No.5,458,832 (issued Oct. 17, 1995); U.S. Pat. No. 5,527,573 (issued Jun.18, 1996); U.S. Pat. No. 6,197,233 (Mar. 6 2001); U.S. Pat. No.6,213,540 (Apr. 10, 2001); and U.S. Pat. No. 6,350,822 (issued Feb. 26,2002), all of which are hereby incorporated herein by reference.

[0075] 5.3.1 Production of Pre-Shaped Core Materials of the Invention

[0076] A typical process is outlined in FIG. 1. The polyester andadditives (“feed mixture”) A are dried B, blended C, and fed into anextruder D via the feed section of the extruder screw. The compositionof the feed mixture is described in more detail in Section 5.3.3 below.Preferably, an extruder hopper (not shown) is used to funnel feedmixture A into extruder D. Feed mixture A may be blended, according towell-known methods, in a separate blender, in the extruder hopper, or inthe extruder itself. Feed mixture A may be dried, according towell-known methods, in a separate drier or in the extruder hopper. Thedrying step may be omitted, for example, if dry raw materials are used.Extruder D, extruder drying equipment B, and extruder cooling equipmentF are well known in the art. Any basic thermoplastic screw-type extrudercan be used. Suitable extruders are described in U.S. Pat. No.3,573,152; (issued Mar. 30, 1972); U.S. Pat. No. 5,340,846 (issued Aug.23, 1994); U.S. Pat. No. 6,254,977 (issued Jul. 3, 2001); and U.S. Pat.No. 6,350,822 (issued Feb. 26, 2002), all of which patents are herebyincorporated by reference herein. Preferably, extruder D comprises asingle-screw, which preferably, has a length to diameter ratio of 44,for example, about 6 cm in diameter and 280 cm in length. Preferably,the extruder has a two-stage foaming screw. Dryer B can be any standarddryer, such as a AEC Whitlock brand dryer (Wood Dale, Ill.) for example,Models WD-25MR and WD-50MR desiccant-bed dryers, which have a capacityof 3.0 and 6.0 cubic feet, respectively. Preferably, cooling apparatus Fis located at the end of the extruder barrel to cool the melt such thata melt strength and viscosity suitable for foaming is achieved. Once inextruder D, the feed mixture is pressurized (typically at a pressure offrom about 25 atmospheres to 200 atmospheres depending on the blowingagent's properties) and melted by heating above the foamable polyester'smelting point in the extruder screw. Then one or more blowing agents Eare transferred into the extruder, for example, via a liquid injectionpump or gas cylinder with injector valves, and blended with the feedmixture under pressure resulting in a polyester/blowing agent mixture(“foamable gel”). Suitable blowing agents are described in more detailin Section 5.3.4 below. The foamable gel is pressurized to preventblowing-agent evaporation. The foamable gel is cooled and metered by thescrew through multi-orifice die plate G. The die plate is described inmore detail in Section 5.3.5 below. Optionally, the mixture may becooled F, for example, in a melt cooler, in a “cooling stage” of theextruder, in a die adapter, or in a secondary extruder. If the foamablegel is too hot or its viscosity too low, upon extrusion, the foam cellswill expand too rapidly, leading to cell-wall rupture and foam collapse.If, on the other hand, the foamable gel is too cool, foaming will besuppressed. Parameters, adjustment, and methods for temperature andpressure control at the foaming stage are well known in the art. Diebody and multi-orifice die plate G are fastened to the forward end ofthe extruder barrel or, if a separate cooling unit is used, then at thedischarge end of the extruder cooler. Upon exiting the die into ambientpressure, the foamable gel foams as a result of blowing-agentevaporation and expansion. Upon foaming, the strands coalesce to give acoalesced multi-stranded pre-shaped core material H. The extrudedstrands require time and space to obtain sufficient expansion, which arereadily determined by one of skill in the art. The temperature rangefrom die plate to shaper including the foaming land (the “land” regionof the die is the smallest diameter of the orifice) should be controlledbased on the blowing agent, nucleating agent, melt viscosity, and meltstrength and the foam's desired physical characteristics according towell-known methods and parameters.

[0077]FIG. 2 is an illustration of pre-shaped core material's 1 strandpattern 2 prior to shaping. The pre-shaped core material comprisescoalesced strands 3 and may comprise inter-strand voids 4 runningparallel to the strand direction.

[0078] An alternative foaming extrusion process, which is well known inthe art, employs tandem extruders. According to this process, thepolyester and other components are mixed, melted, pressurized, andhomogenized with the blowing agent(s) in a first extruder, as describedabove, resulting in the foamable gel. The foamable gel is transferred bymeans of a conventional transfer tube or static mixer, optionallyassisted by a gear-type melt pump, to the feed section of the secondextruder, which conveys the foamable gel to the die and adjusts thetemperature and pressure necessary for optimum foaming. Typically, thetwo extruders are of different sizes. The well-known tandem-extruderprocess allows excellent control of process variables. See e.g., JAMESL. THRONE, THERMOPLASTIC FOAMS 191 (1st ed. 1996); K.-D. Kolossow,Extrusion of Foamed Intermediate Products With Single-Screw Extruders,in PLASTICS EXTRUSION TECHNOLOGY 456 (F. Hensen ed. 1988), both of whichtitles are incorporated by reference herein.

[0079]5.3.2 Shaping the Extruded Pre-Shaped Core Materials of theInvention

[0080] The special shaping process described below serves threefunctions: (a) to compress the pre-shaped core material's coalescedstrands and eliminate inter-strand voids between and running parallel tothe strands; (b) To mold the core material of the invention to thedesired shape; and (c) to provide the core material with a smoothsurface. The shaper of the invention basically comprises a shapingconduit of specified shape, having an entrance, an exit, and a length,with the conduit's top having a degree of decline, while the angle ofthe sides and bottom remain straight (i.e., the sides and bottom arefixed in a parallel relationship). As the recently extrudedmulti-stranded core material is forced or pulled through the shaper, itis compressed. The compression removes the inter-strand voids resultingin the high-strength core materials of the invention (see FIG. 4 showinga cross section of a core material of the invention having nointer-strand voids). The shaping process can be tailored by adjustingthe entrance and exit size, length, and the degree of decline tocompletely remove the strand appearance of the core material. That is,if the core material is cut perpendicular to the strand direction,strands are no longer visible or defined (at least to the human eye).The strands have coalesced to a degree wherein the core material appearsto be unitary. But surprisingly, this shaping results in a plurality ofdiscrete volumes. These discrete volumes are distinguished from oneanother in that each comprises a core having closed cells of averagecell size relatively larger than the average cell size of the cells in asurrounding jacket. This phenomenon is believed to contribute to theanisotropic properties of the core materials of the invention and ismore fully discussed in Section 5.4.1 below.

[0081] In any case, in next step of FIG. 1, the pre-shaped core materialH is conveyed to shaper I, for example, via a puller (not shown) orsimilar machine. Shaper I compresses the pre-shaped core material, togive a core material of the invention J. Shaper I is placed adjacent todie plate G, the distance between the die plate and shaper may beadjusted according to the strength, temperature, and expansion offoaming strands.

[0082]FIGS. 3A and 3C illustrate a shaper 10, suitable for use in theinvention. FIG. 3A illustrates a perspective view; a side view is shownin FIG. 3B; and a front view is shown in FIG. 3C. The shaper comprises afixed channel-shaped bottom 12 connected via a plurality of evenlyspaced spring bolts 14 to adjustable top cap 15, having angle of decline16. As shown in FIG. 3C, top cap 15 is shaped for a close fit into thechannel defined by bottom 12 thereby defining shaping conduit 17, havingentrance 18 and exit 19. The position of top cap 15 and its angle ofdecline 16 can be adjusted with spring bolts 14. Upon reading theSpecification, one of skill in the art will understand that theparameters of exit size, entrance size, shaping conduit length, and theshaping conduit top's angle of decline depend on the dimensions andpattern of the die plate, the extrusion parameters, blowing agent,polyester resin etc. Preferably, these parameters are adjusted such thatall the voids between the strands are substantially removed.Substantially removed means that the inter-strand voids are eliminatedto the extent that they are not visible to the human eye or are ofdiameter smaller than the average cell-size diameter. Preferably, thecross area of entrance 18 is of from about 3.0 times to about 1.2 timesthe cross area of exit 19, more preferably, of from about 2.5 times toabout 1.5 times, even more preferably, of from about 2.0 times to about1.5 times. The length of the shaper is adjusted based on foam residencetimes. Thus, shaper length is based on speed of processing, which inturn is dependent on other variables, such as polyester identity, cellsize, blowing agent, etc., all of which variables are readily determinedand adjusted by one of ordinary skill in the art. Preferably, the lengthof the shaper is about 1 ft. and the foam has a residence time in theshaper of about 10 to about 30 seconds. Preferably, the angle of decline16 of the shaping conduit's top 15 is of from about 20° to about 2°,more preferably, of from about 15° to about 5°, even more preferably, offrom about 10° to about 5°. Bottom 12 is fixed to support 20, such as abench, by strips 22. Preferably, strips 22 are constructed of steel.

[0083] As shown in FIGS. 3A and 3B, the distance between the top andbottom of shaping conduit 17 gradually decreases while the width remainsfixed, thus, when a pre-shaped core material is pulled through shapingconduit 17, it is compressed in a direction perpendicular to its length.The distance between the top and bottom of shaping conduit 17 iscontrolled by the position of top cap 15. The exterior force required toadjust top cap 15 can be applied by well-known methods, such as gravity,spring-loaded bolts 14, or air pressure from a self-adjusting system.Preferably, shaper 10 is equipped with oil lines 24 parallel to theshaper's length, situated in the lower wall of bottom 12. The oiltemperature is adjusted by a heat-exchange system and pumped throughlines 24 to control the shaping temperature. One of skill in the artwill adjust the shaping temperature considering the identity anddimensions of the foamed strands and the blowing agent's properties. Theshaper can be constructed of stainless steel, aluminum, copper, orgraphite plates, preferably, aluminum. Preferably, the interior surfacesof shaping conduit 17 are coated with a lubricant or other non-stickcoating to reduce the refraction force between the pre-shaped corematerial and the shaper walls, for example, but not limited to,fluoropolymers (e.g., Lubricating Spray Coating, a product ofSaint-Gobain Performance Plastics, Wayne, N.J.).

[0084] FIGS. 3A-3C illustrate a shaper of the invention that gives arectangular core material of the invention. By changing die-platepattern and the shaper's configuration, different configurations of thecore materials of the invention can be achieved, for example, U-channel,I-beam, V, trapezoid, rod, and pipe shapes. The distance from the shaperentrance to the die plate can be adjusted by one of skill in the artdepending on the foamable polyester resin, blowing agent(s), anddie-plate configuration. Preferably, the shaped core material of theinvention has a smooth surface and substantially no voids between thestrands.

[0085]FIG. 4 illustrates a cross sectional view of a typical corematerial 30 of the invention after the special shaping process. Asindicated in FIG. 4 by the use of dotted lines, the strand interfacelines 32 are barely visible, or depending on the extrusion and shapingparameters, not visible at all and no inter-strand voids are present.The shaped core materials of the invention can be cut into desireddimensions by well-known methods.

[0086] 5.3.3 Feed Mixture, Nucleating Agents and Other Additives

[0087] Preferably, the feed mixture comprises foamable polyester and theother additives if they will be used. Preferably, a nucleating agent isincluded in the feed mixture to promote even evaporation of the blowingagent, thereby controlling the size and number of cells and cell-sizedistribution. Nucleating agents are well known in the art. Suitablenucleating agents include, but are not limited to, inorganic substances,such as calcium carbonate, talc, clay, titanium oxide, silica, bariumsulfate, diatomaceous earth, and carbon dioxide that is generated byincluding a mixture of a basic salt (e.g., sodium-, potassium-, orammonium carbonate or bicarbonate) and an inorganic or organic acid(e.g., boric acid, citric acid, and tartaric acid) in the feed material.Finely pulverized inorganic substances, such as calcium carbonate andtalc are preferred. The particle size of nucleating agent is from 0.1 to20 microns, preferably, from 1 to 3 microns. Preferably, the nucleatingagent is present in an amount of from about 0.01% to about 5% by weightof polyester, more preferably, of from about 0.1% to about 3% by weight.In general, more nucleating agent gives a smaller average cell diameter.If the amount exceeds 5% by weight, agglomeration or insufficientdispersion of nucleating substance occurs, which results inover-expanded cells and often cell collapse. On the other hand, if thenucleating agent is present in an amount of less than about 0.01% byweight, the nucleating action is negligible.

[0088] In another embodiment, the polyester can be non-foamablepolyester having one or more branching agents included as an additive torender the polyester foamable in situ. Upon feed-mixture heating andmelting, the polyester and branching agent react, in situ, to givefoamable polyester, which is extruded as above. Such a process isdescribed in U.S. Pat. No. 5,340,846 (issued Aug. 23, 1994) and U.S.Pat. No. 6,254,977, (issued Jul. 3, 2001) both of which are herebyincorporated by reference herein.

[0089] Optionally, the feed mixture may comprise further additives, aswell known in the art, depending on the desired properties of the corematerial. Examples of other additives include, but are not limited toreclaim polymer generated in manufacturing, flame retardants, colorantsor pigments, anti-static agents, antioxidants, ultraviolet rayabsorbents, and reinforcement by short fibers, etc.

[0090] 5.3.4 Blowing Agents

[0091] The blowing agents suitable for use in the invention generallyhave a boiling point temperature range of about −90° C. to about 130° C.Suitable blowing agents include, but are not limited to, aliphatichydrocarbons, such as octane, heptane, hexane, cyclohexane, pentane,cyclopentane, isopentane, neo-pentane, isobutane, butane, propane, andethane; alcohols, such as methanol, ethanol, isopropanol, and butanol;non-fully chlorinated chlorohydrocarbons, partially or fully fluorinatedfluorohydrocarbons, and non-fully halogenated fluorochlorohydrocarbons,such as 1-chloro-1,1-fluoroethane; 1,1,1,2-tetrafluoroethane (HFC 134a);1,1-difluoroethane (HFC 152a); 1-chloro-1,1-difluoroethane (HCFC 142b);1,1,1,3,3-pentafluorobutane (HFC-365 mfc); 1,1,1,3,3-pentafluoropropane(HFC-245fa); gases, such as carbon dioxide, argon, and nitrogen; andcompounds that decompose in situ to release a blowing-agent gas, such asazobisformamide or azodicarbonamide. Preferably, the blowing agent ishexane, carbon dioxide, or 1,1,1,2-tetrafluoroethane or1,1-difluoroethane.

[0092] The amount of blowing agent depends on the desired foam densityand such amounts are readily determined by well-known methods, such asthose described in U.S. Pat. No. 5,681,865 (issued Oct. 28, 1997),hereby incorporated herein by reference. The amount of blowing agentshould not exceed an amount that causes separation between the polyestermelt and blowing agent in the extruder, that is exceeds the solubilityof said blowing agent in the melt at operating temperatures. Thepreferred amount of blowing agent is of from about 0.5% by weight toabout 15% by weight of the feed mixture, more preferably, of from about1% to about 5%, most preferably, of from about 1% to about 3%. Mixturesof blowing agents can be employed, such as the mixtures described inU.S. Pat. No. 5,679,295 (issued Oct. 21, 1997) according to theprocedure described therein, hereby incorporated by reference herein.For example, the following combination of blowing agents could be used:(1) of from about 50 mole percent to about 99.9 mole percent of a firstblowing agent having a boiling temperature at STP (Standard Temperatureand Pressure) of greater than 310° K., such as heptane, octane, orcyclopentane; (2) of from about 0.1 mole percent to about 50 molepercent of a second blowing agent having a boiling temperature at STP ofless than 310° K., such as butane, tetrafluoroethane, carbon dioxide, orpentane.

[0093] 5.3.5 Die Plate

[0094] Foam strands that exit from the orifice generally expand to about3 to about 6 times the orifice diameter depending on the density, thusthe ratio of total orifice cross-sectional area to that of the interiorof the die just ahead of the orifice-containing faceplate should be lessthan 10 percent, more preferably, less than 5 percent.

[0095] The die plate comprises a plurality of orifices, in the shape ofholes, slits, or any other desired shape, such as square saw tooth ortriangular saw tooth wave pattern. The size of the holes can becalculated based on a method described in U.S. Pat. No. 6,197,233(issued Mar. 6, 2001), hereby incorporated by reference herein. Theratio of the theoretical diameter of an individual strand to thedistance between the orifices is, preferably, greater than or equal to1, more preferably, greater or equal to 1.2. The theoretical diameter ofeach strand can be calculated based on the foam volume expansion andextrusion-direction speed of the product. The foam volume can becalculated from mass balance and foaming temperature. The orifice sizeand overall open area are determined also by considering extrudethroughput and foaming pressure. The big overall open area can increasethroughput and foam cross section area, but reduce the die pressure andcause foaming in die. The only limitation on special arrangement of thedie orifices is that contact and coalescence of adjacent strands orprofiles after extrusion from the die faceplate must be achieved.

[0096] Merely by way of example, a suitable die plate has the followingconstruction: a diameter of orifice is about 1.5 mm, wherein theorifices are arranged in a triangular pattern with a distance of about6.5 mm between the center of the orifices. In another embodiment, apreferred die-faceplate pattern comprises evenly spaced circularorifices of a diameter of from about 0.8 mm to about 4 mm, preferably,of from about 1.5 mm to about, more preferably, of from about 1 mm toabout 2 mm. For example, a suitable die is a circular carbon steel plateof a diameter of 165 mm comprising about 34 equally spacedcircular-shaped holes, contained in an area of 7 cm×2 cm, arranged infour rows, wherein the holes are about 0.065 inches (1.65 mm) indiameter and spaced approximately 0.25 inches (6.35 mm) between thecenters. In another example, a suitable die is a circular carbon steelplate of a diameter of 165 mm comprising about 34 equally spacedcircular-shaped holes, contained in an area of 7 cm×2 cm, arranged infour rows, wherein the holes are about 0.052 inches (1.32 mm) indiameter and spaced approximately 0.25 inches (6.35 mm) between thecenters. In yet another example, a suitable die is a circular carbonsteel plate of a diameter of 165 mm comprising about 78 equally spacedcircular-shaped holes contained in an area of 10 cm×2.5 cm arranged infive rows, wherein the holes are about 0.065 inches (1.62 mm) indiameter and spaced approximately 0.25 inches (6.35 mm) between thecenters. Special alloys of more highly conductive metals, such asberyllium-copper, and having heat-transfer fluid passages or electricheaters, would be advantageous in the precise control of temperature ofthe die faceplate.

5.4 Characteristics of Core Materials of the Invention

[0097] Core materials of the invention are chemically and thermallystable by virtue of the polyesters from which they are derived.Furthermore, by virtue of the preparation methods described herein, thecore materials of the invention are low-density, relatively small-cellsized, closed-cell foams, of uniform cell size with a low open-cellcount. The core materials of the invention are further characterized byexcellent mechanical properties, such as high compression strength andcompression modulus, high shear strength and shear modulus, and hightensile strength and tensile modulus. Strength refers to the maximumload per unit of area transverse to the loading direction, that thematerial can resist. Modulus is precise measurement of how muchdeformation the material will exhibit under a given load. High modulusmeans that the structure deforms relatively little under large appliedforces.

[0098] 5.4.1 Inter-Strand Cell-Size Distribution

[0099] As discussed above, in the shaping process, as the recentlyextruded multi-stranded core material is forced or pulled through theshaper; it is compressed, thereby removing the inter-strand voids, suchthat the core materials of the invention appear to be unitary (thestrand structure is not visible to the human eye). The properties,however, of the unitary polyester core materials of the invention aremuch improved over traditional unitary block foams produced by extrudingthrough a single-aperture die or unshaped multi-stranded foams. Theorigins of these improved properties, such as strength, are not fullyunderstood, but may be due to the change in cell-size distribution thataccompanies extrusion and shaping of multi-stranded polyester foams.Upon extrusion and prior to shaping, the core material comprises amultitude of coalesced foamed strands having inter-strand voids runningparallel to the strand direction. Upon shaping, the inter-strand voidsare removed, and the core materials of the invention often appearunitary, although, in some cases, the strand intersections can be seen.

[0100] Surprisingly, however, a plurality of “discrete volumes”, runningparallel to the length of the core material results. These “discretevolumes” comprise an “interior section” running parallel to the stranddirection and a “jacket” surrounding the “interior section of thediscrete volume”. This interior section has closed cells of average cellsize relatively larger than the average cell size of the cells in thesurrounding “jacket”. In other words, the average-cell diameter issmaller where the strands have intersected and merged than at theoriginal strand's interior. Accordingly, as used herein, the phrase“interior section of the discrete volume” means a section within a corematerial of the invention running in the strand direction that issurrounded by a “jacket”. For example, the “jacket” could be a donutshaped cylindrical volume and the interior section of the discretevolume a cylindrical core inside the cylindrical donut. It follows thatthe sum value of the volume of the “interior section of the discretevolume” and the volume of the “jacket” equals the “discrete volume”,which discrete volume approximates the volume of the correspondingstrand before shaping.

[0101] Where the interior section of the discrete volume approximates acylinder shape, such a cylinder has a cross-sectional area ofπ(x)(1/2d)² where “d” is the diameter of the discrete volume. Preferably“x” is a value of from about 0.99 to about 0.01, more preferably, offrom about 0.95 to about 0.5, even more preferably, of from about 0.90to about 0.75. The volume and shape of the “jacket” surrounding the“interior section of the discrete volume” is the difference between thediscrete volume and the volume of the interior section of the discretevolume. Suitable extrapolations can be made by one of skill in the artwhen the interior section does not approximate a cylinder. Preferably,the average cell diameter of cells in the jacket of the discrete volumeis of from about 5% to about 98% of the average cell diameter of cellsin the interior section of the discrete volume, more preferably, of fromabout 20% to about 80%, even more preferably, of from about 30% to about70%. This inter-strand cell-size distribution can be viewed by anysuitable technique, such as optical microscope techniques so as todistinguish cell diameters within the plurality of discrete volumes.

[0102] 5.4.2 Anisotropic Properties of Core Materials of the Invention

[0103] The core materials of the invention are significantly anisotropicin character; meaning that they exhibit different physical propertiesdepending upon which axis (direction) the property is measured. Forexample, the core materials of the invention exhibit different shearproperties, tensile properties, and compressive properties dependingupon the axis along which the property was measured. This character isrelated to the fact that the core materials of the invention are derivedfrom compressed strands. Although the strand character of the corematerials of the invention may not be visible, the anisotropic characteris readily apparent through standard testing, for example, testing fordifferences in shear properties versus applied-force direction, tensileproperties versus applied-force direction, and compressive propertiesversus applied-force direction. For example, in a core material of theinvention, the shear strength in the end-strand direction differs fromthe shear strength in the transverse direction, which both differ fromthe shear strength in the longitudinal direction.

[0104] In core materials of the invention, the shear strength differs,the shear modulus differs, and the shear elongation at break differsdepending on the relationship between the applied-force direction andthe strand direction. Preferably the shear strength difference rangesfrom about 10% to about 400% higher in the end-strand over thetransverse direction, more preferably, from about 20% to about 200%,even more preferably, from about 40% to about 150%. Preferably, theshear modulus difference ranges from about 10% to about 400% higher inthe end-strand over the transverse direction, more preferably, fromabout 20% to about 200%, even more preferably, from about 40% to about150%. Preferably the shear elongation at break point difference rangesfrom about 10% to about 600% higher in the end-strand over thetransverse direction, more preferably, from about 20% to about 400% evenmore preferably, from about 50% to about 200%.

[0105] In core materials of the invention, the tensile strength differsand the tensile modulus differs depending on the relationship betweenthe applied-force direction and the strand direction. Preferably thetensile strength difference ranges from about 100% to about 3000% higherin the end-strand over the transverse direction, more preferably, fromabout 500% to about 2000%, even more preferably, from about 800% toabout 1100%. Preferably the tensile modulus difference ranges from about100% to about 3000% higher in the end-strand over the transversedirection, more preferably, from about 500% to about 2000%, even morepreferably, from about 900% to about 1500%.

[0106] In core materials of the invention, the compressive strengthdiffers and the compressive modulus differs depending on therelationship between the applied-force direction and the stranddirection. Preferably the compressive strength difference ranges fromabout 50% to about 1000% higher in the end-strand over the transversedirection, more preferably, from about 100% to about 500%, even morepreferably, from about 120% to about 300%. Preferably the compressionmodulus difference ranges from about 10% to about 500% higher in theend-strand over the transverse direction, more preferably, from about20% to about 200%, even more preferably, from about 30% to about 100%.

[0107] 5.4.3 Density

[0108] The core materials of the invention are characterized by lowdensity, preferably, of from about 1 kg/m³ to about 400 kg/m³, morepreferably, of from about 50 kg/m³ to about 300 kg/m³, and mostpreferably, of from about 60 kg/m³ to about 250 kg/m³ as measuredaccording to ASTM Test Method D1622-98 “Standard Test Methodfor ApparentDensity of Rigid Cellular Plastics”, hereby incorporated by referenceherein. This publication sets forth procedures for the determination ofboth the apparent overall density and the apparent core density ofcellular plastics.

[0109] 5.4.4 Average Cell Size

[0110] The core materials of the invention are characterized by smallaverage cell size, preferably, of from about 0.05 mm to about 1.5 mm,more preferably, of from about 0.1 mm to about 0.5 mm, as measuredaccording to ASTM Test Method D3576-98 “Standard Test Methodfor CellSize of Rigid Cellular Plastics”, hereby incorporated by referenceherein. This publication sets forth methods for determination of theapparent cell size of rigid cellular plastics by counting the number ofcell-wall intersections in a specified distance. Procedure A requiresthe preparation of a thin slice, not more than one half the average celldiameter in thickness, that is mechanically stable. For most rigidcellular plastics this limits the test method to materials with anaverage cell size of at least 0.2 mm. Procedure B is intended for usewith materials whose friable nature makes it difficult to obtain a thinslice for viewing.

[0111] 5.4.5 Open-Cell Content

[0112] The core materials of the invention are characterized by a lowopen-cell content, preferably, of from about 5 to about 20, morepreferably, of from about 5 to about 10 as measured according to ASTMTest Method D2856-94 (1998) “Standard Test Method for Open-Cell Contentof Rigid Cellular Plastics by the Air Pycnometer”, hereby incorporatedby reference herein. Cellular plastics are composed of the membranes orwalls of polymer separating small cavities or cells. These cells may beinterconnecting (open cell), non-connecting (closed cell), or anycombination of these types. This test method determines numerical valuesfor open cells. It is a porosity determination, measuring the accessiblecellular volume of a material. The volume occupied by closed cells isconsidered to include cell walls. This test method consists of threeprocedures: procedure A, designed to correct for cells opened duringsample preparation, by measuring cell diameter, calculating, andallowing for surface volume; procedure B, designed to correct for cellsopened in sample preparation, by cutting and exposing new surface areaequal to the surface area of the original sample dimension; andprocedure C, which does not correct for cells opened during samplepreparation and gives good accuracy on predominantly highly open-celledmaterials. The accuracy decreases as the closed-cell content increasesand as the cell size increases.

[0113] 5.4.6 Compression Strength and Compression Modulus

[0114] High compression strength and medium compression modulus in thestrand direction characterize the core materials of the invention. Forexample, core materials of the invention, preferably, have a compressionstrength of from about 0.20 Mpa to about 1.5 Mpa, more preferably, offrom about 1.0 to about 1.5 Mpa at a density of 80 kg/m³. At a densityof 140 kg/m³, preferably, the core materials have a compression strengthof from about 0.8 Mpa to about 2.5 Mpa, more preferably, of from about1.5 Mpa to about 2.5 Mpa. Preferably, core materials of the inventionhave a compression modulus of from about 10 Mpa to about 30 Mpa, morepreferably, of from about 15 Mpa to about 25 Mpa at a density of 80kg/m³. At a density of 140 kg/m³, preferably, the core materials have acompression modulus of from about 25 Mpa to about 65 Mpa, morepreferably, of from about 45 Mpa to about 55 Mpa. Compression propertiescan be measured according to the procedure set forth in ASTM Test MethodC365-00 “Standard Test Methodfor Flatwise Compressive Properties ofSandwich Cores”, hereby incorporated by reference herein. Thispublication describes determination of the compressive strength andmodulus of composite cores. These properties are usually determined fordesign purposes, in a direction normal to the plane of facings as thecore would be placed in a structural sandwich construction. The testprocedures pertain to compression in this direction in particular, butalso can be applied with possible minor variations to determiningcompressive properties in other directions.

[0115] 5.4.7 Tensile Strength and Tensile Modulus

[0116] The core materials of the invention are characterized by mediumtensile strength and high tensile modulus. Tensile strength of the corematerial provides and indication of its composite's resistance to skinbuckling upon edgewise loading. For example, core materials of theinvention, preferably, have a tensile strength of from about 0.5 Mpa toabout 2.5 Mpa, more preferably, of from about 1 Mpa to about 2 Mpa at adensity of 80 kg/m³. At a density of 140 kg/m³, the core materials,preferably, have a tensile strength of from about 1.5 Mpa to about 3.5Mpa, more preferably, of from about 2 Mpa to about 3 Mpa. Preferably,core materials of the invention have a tensile modulus of from about 50Mpa to about 200 Mpa, more preferably, of from about 100 Mpa to about150 Mpa at a density of 80 kg/m³. At a density of 140 kg/m³, the corematerials, preferably, have a tensile modulus of from about 100 Mpa toabout 300 Mpa, more preferably, of from about 150 Mpa to about 250 Mpa.The tensile strength and modulus can be measured according to theprocedure set forth in ASTM Test Method C297-94 “Standard Test MethodforFlatwise Tensile Strength of Sandwich Constructions”, herebyincorporated by reference herein. This publication illustrates thedetermination of the core flatwise tension strength, or the bond betweencore and facings of an assembled sandwich panel. The test consists ofsubjecting a sandwich construction to a tensile load normal to the planeof the sandwich, such load being transmitted to the sandwich throughthick loading blocks bonded to the sandwich facings or directly to thecore.

[0117] 5.4.8 Shear Strength and Shear Modulus

[0118] The core materials of the invention are characterized by mediumshear strength and high shear modulus. For example, core materials ofthe invention, preferably, have a shear strength of from about 0.3 MPato about 1.5 Mpa, more preferably, of from about 0.6 MPa to about 1.2Mpa at a density of 80 kg/m³. At a density of 140 kg/m³, the corematerials, preferably, have a shear strength of from about 0.5 MPa toabout 3 Mpa, more preferably, of from about 1 Mpa to about 2 Mpa.Preferably, core materials of the invention have a shear modulus of fromabout 10 to about 35, more preferably, of from about 20 MPa to about 30MPa at a density of 80 kg/m³. At a density of 140 kg/m³, the corematerials, preferably, have a shear modulus of from about 20 Mpa toabout 60 MPa, more preferably, of from about 30 MPa to about 50 Mpa. Theshear strength and modulus can be measured according to the procedureset forth in ASTM Test Method C273-00e1 “Standard Test Method for ShearProperties of Sandwich Core Materials”. This publication illustrates thedetermination of shear properties of sandwich construction corematerials associated with shear distortion of planes parallel to thefacings. It describes determination of shear strength parallel to theplane of the sandwich, and the shear modulus associated with strains ina plane normal to the facings. The test may be conducted on corematerials bonded directly to the loading plates or the sandwich facingsbonded to the plates.

5.5 Composites of the Invention

[0119] Composites of the invention can be prepared according towell-known methods by integrally bonding the core materials of theinvention to standard composite skins. Examples of such methods aredescribed in U.S. Pat. No. 6,206,669 (issued Mar. 27, 2001); U.S. Pat.No. 6,156,146 (issued Dec. 5, 2000); U.S. Pat. No. 6,117,519 (issuedSept. 12, 2000); U.S. Pat. No. 6,013,213 (issued Jan. 11, 2000); U.S.Pat. No. 5,916,672 (issued Jun. 29, 1999); U.S. Pat. No. 5,904,972(issued May 18, 1999); U.S. Pat. No. 5,580,502 (issued Dec. 3, 1996);and U.S. Pat. No. 5,316,462 (May 31 1994), all of which are herebyincorporated by reference herein. Just about any composite skin suitablefor use with thermoplastic foam cores can be used. Such composite skinsare commercially available, for example, from M. C. Gill (El Monte,Calif.), DFI Pultrude Composites, Inc. (Erlanger, Ky.); and GordonPlastics (Montrose, Calif.). Examples of suitable composite skinsinclude, but are not limited to, thermoplastic polymers andthermosetting polymers, optionally reinforced with glass fibers,metallic fibers, inorganic fibers, or carbon fibers; wood; inorganicmaterials, such as fiberglass; and metallic materials, such as aluminumand stainless steel and many others, which are well known to those ofskill in the art. The composites can be built up in layers. Whenbuilding composites in layers one or more structural skins can compriseanother structural sandwich composite.

[0120] A preferred composite-manufacture method is resin infusionmolding, which comprises vacuum suction to pull liquid resin into a drylay up (composite skins and core materials) with almost nohazardous-vapor emissions. The chemically and thermally resistant corematerials of the invention are especially suited to such processing.

[0121] In some applications where it is necessary that the core of thelaminate be conformed to a curved surface, such as a boat hull or acylindrical storage tank, the core materials of the invention can beprocessed into a contourable blankets or web-like structures, whereinthe core material of the invention is divided into an array of smallerblocks or tiles. Such a blanket may be adhered to a fabric scrim orcommon carrier whereby the blanket can be conformed to a contouredsurface. A process for producing contourable blankets from foamedarticles is disclosed in U.S. Pat. No. 5,798,160 (issued Aug. 25, 1998),hereby incorporated herein by reference. The contourable blanketstructures are thereafter bonded to skins to form contoured compositematerials of the invention.

[0122] To produce contourable core materials of the invention, severalstandard slabs are stacked and bonded together by thermal fusion or by asuitable adhesive to create a large multi-slab block. The block is thensliced transversely in parallel planes normal to the lines of adhesiveto yield a plurality of panels. A saw can be used for slicing. Eachpanel is composed of a series of interconnected foam-plastic sectionsderived from respective slabs of the block whereby the density of thesections is evenly distributed throughout the panel and the mechanicalproperties of the panel are therefore predictable and satisfy structurallaminate criteria.

[0123] In the basic process for producing composite materials of theinvention, the core materials of the invention (in rigid,contourable-blanket form, or any other form) and skins are laid up in atool while dry then a vacuum bag is placed over the lay up and sealed tothe tool. Resin is introduced via a resin inlet and distributedthroughout the laminate. The pressure differential provides the drivingforce for infusing the resin into the lay up. The procedure is describedin detail in U.S. Pat. No. 4,902,215 (issued Feb. 20, 1990); U.S. Pat.No. 5,052,906 (issued Oct. 1, 1991); U.S. Pat. No. 5,721,034 (issuedFeb. 24, 198); U.S. Pat. No. 5,904,972 (issued May 18, 1999); U.S. Pat.No. 5,958,325 (issued Sept. 28, 1999); and U.S. Pat. No. 6,159,414(issued Dec. 12, 2000), all of which are hereby incorporated herein byreference. Vacuum-bag lay-up products are commercially available, forexample, from Airtech International (Carson, Calif.); HawkeyeEnterprises (Los Angeles, Calif.); National Aerospace Supply Co. (SanClemente, Calif.); Richmond Aircraft Supply (Norwalk, Calif.) andTaconic, Process Materials Division (Santa Maria, Calif.). Resins forbonding the skins to the core materials of the invention arecommercially available, for example, from Aircraft Spruce & SpecialtyCompany (Corona, Calif.); CMI/Composite Materials Inc. (Santa FeSprings); E. V. Roberts (Culver City, Calif.); Gougen Brothers, Inc.(Bay City, Mich.); and National Aerospace Supply Co. (San Clemente,Calif.).

6. EXAMPLES 6.1 Example 1

[0124] A mixture of polyethylene terephthalate resin (COBIFOAM 0,purchased from M&G Polymers, Patrica, Italy) having an intrinsicviscosity of 1.25 dl/g; and a melting point of 251° C. and nucleatingagent was dried for 5 hours at 330° F. (165° C.) by a dehumidifieddryer. The mixture was charged into a two-stage extruder having a 2.5″(6.35 cm) single-screw, length to diameter of 44 to 1, with a SMR meltcooler heated to a temperature of 280° C.; and pressurized to 10 Mpa, apolyethylene terephthalate melt. Blowing agent (3 weight percent of thetotal weight of the melt) was injected by a injection pump and themixture homogenized in the extruder by mixing section, static mixer andSMR melt cooler to give a foamable gel. The temperature was reduced to250-255° C. at a pressure of 3.5-5.0 Mpa and the foamable gel wasextruded through a rectangular multi-orifice die plate that was fastenedon the die body, which the other end of the die body was attached to themelt cooler. The extrusion rate was 20 kg/hr. The die plate was 7.0cm×2.0 cm and comprised 34 equally spaced circular-shaped holes arrangedin four rows. The holes were approximately 0.065 inches (1.65 mm) indiameter and spaced approximately 0.25 inches (6.35 mm) between thecenters.

[0125] The following parameter values were used in the extrusion.Parameter Value diameter of screw 6.35 cm screw-length to diameter 44:1temperature of melting zone   280° C. temperature of injection zone  280° C. temperature of cooling zone 270-275° C. head temperature255-260° C. temperature of the melt 255-260° C. pressure of the melt3.5-5.0 Mpa runs of the screw 15 rpm average residence time in theextruder 15 min

[0126] The resulting coalesced multi-stranded pre-shaped core materialwas conveyed through a shaper of the invention, constructed of aluminumplate, having a width of 7 cm and a length of 30 cm. The ratio of theopening area to the parallel cross-sectional area was 2.3. The first 2.5cm range of shaper has a 12° angle of decline so that the expandedstrands can be squeezed and compressed into foam board without voidsbetween strands. The distance of the cap plate to the bottom of shaperwas adjusted from 3 cm to 4 cm based on output of extrusion and speed oftake off equipment.

[0127] The resulting core material of the invention had a cross sectionof 7 cm by 3.4 cm; uniform cell size (<0.2 mm); no voids between thestrands; and a foam density of 5.4 pcf (86.5 kg/m³) pounds per cubicfoot. The core material so produced had the following characteristics:Characteristic Value Density 86.5 kg/m³ Average cell size  0.1 mmOpen-cell content <10%

6.2 Example 2

[0128] The procedure of Example 1 was followed using 4 weight percentblowing agent and a die plate of 7.0 cm×2.0 cm comprising 34 holes eachof 0.052 inches (1.32 mm) in diameter. The holes and rows were equallyspaced at approximately 0.25 inches (6.35 mm) between the centers. Thecoalesced multi-stranded pre-shaped core material was shaped asdescribed in Example 1. The resulting core material of the invention hada cross section of 2.75 inches (7.0 cm) by 1.45 inches (3.68 cm);uniform cell size (<0.3 mm); no voids between the strands; and a foamdensity of 4.7 pounds per cubic foot (75 kg/m³). The core material soproduced had the following characteristics: Characteristic Value Density75 kg/m³ Average cell size  0.2 mm Open-cell content <10%

6.3 Example 3

[0129] The procedure of Example 1 was followed, with 0.15 weight percenttalc as a nucleating agent and 2 weight percent blowing agent. Theresulting core material of the invention had a cross-section of 7.0 cmby 3.18 cm; a uniform cell size (<0.2 mm); no voids between the strands;and a foam density of 7.56 pounds per cubic foot (121 kg/m³). The corematerial so produced had the following characteristics: CharacteristicValue Density 121 kg/m³ Average cell size  0.1 mm Open-cell content <10%

6.4 Example 4

[0130] Using the procedure of Example 1, a foamable gel comprising amixture of polyethylene terephthalate polyester resin (CRYSTAR 5067,available from Du Pont Polyester) having a melting point of 251° C. and1.5 weight percent 1,1,1,2-tetrafluoroethane (HFC R134a) as a blowingagent was cooled to a temperature of 255° C. and extruded through thedie plate. The SMR melt cooler was not used. The die plate was connecteddirectly to the end of extruder; was 10 cm×2.5 cm; and comprised 78equally spaced circular-shaped holes arranged in five rows. The holeswere about 0.065 inches (1.65 mm) in diameter and spaced about 0.25inches (6.35 mm) between their centers.

[0131] The resulting coalesced multi-stranded pre-shaped core materialwas conveyed through an aluminum-plate shaper of the invention having awidth of 11 cm and a length of 30 cm. The ratio of the opening area tothe parallel cross-sectional area was 1.6. The first 2.5 cm range ofshaper has a 12° angle of decline so that the expanded strands can besqueezed and compressed into foam plank without voids between strands.The distance of the cap plate to the bottom of shaper was adjusted from3 cm to 4 cm based on output of extrusion and speed of take offequipment.

[0132] The resulting core material of the invention had a cross sectionof 4.4 inches (11.2 cm) by 1.35 inches (3.4 cm), a uniform cell size(<0.01 mm), and no voids between the strands. The density was 9.5 poundsper cubic foot (152 kg/m³). The core material so produced had thefollowing characteristics: Characteristic Value Density 152 kg/m³Average cell size  0.3 mm Open-cell content <20%

6.5 Example 5

[0133] A mixture of polyethylene terephthalate resin (COBIFOAM 0,purchased from M&G Polymers, Patrica, Italy) having an intrinsicviscosity of 1.25 dl/g and a melting point of 251° C. and talc powder0.3 weight %, was dried for 6 hours at 165° C. by a dehumidified dryer.The mixture was charged into a co-rotating twin-screw extruder, model BC132 PET-F (BC Foam, Volpiano, Italy) equipped with a melt cooler andhomogenizer. The extruder screw was 13.2 cm in diameter and 220 cm inlength. The mixture was heated in the extruder to a temperature of 285°C. and pressurized to 5 Mpa, then 1,1-difluoroethane (R152 A) (1.4weight percent of the total weight of the melt) was introduced via aninjection pump. The mixture was homogenized to give a foamable gel. Thetemperature of the foamable gel was reduced to about 250-255° C. and thepressure was adjusted to about 3.5-4.0 Mpa. The foamable gel wasextruded at a rate of 180 kg/hr through a multi-orifice die plate of 42cm×5 cm comprising 580 equally spaced circular-shaped holes arranged ineight rows. The holes were approximately 1.65 mm in diameter and spacedapproximately 6.35 mm between centers.

[0134] The following parameter values were used in the extrusion:Parameter Value diameter of screw 13.2 cm screw-length 220 cmtemperature of melting zone   285° C. temperature of injection zone  285° C. temperature of cooling zone 265-270° C. head temperature250-255° C. temperature of the melt 250-255° C. pressure of the melt3.5-4.0 Mpa runs of the screw 15 rpm average residence time in theextruder 15 min

[0135] The resulting coalesced multi-stranded pre-shaped core materialwas conveyed through an aluminum-plate shaper of the invention having awidth of 43 cm and a length of 30 cm. The ratio of the opening area tothe parallel cross-sectional area was 1.6. The first 2.5 cm range ofshaper has a 12° angle of decline so that the expanded strands can besqueezed and compressed to remove inter-strand voids. The distance ofthe cap plate to the bottom of shaper was adjusted from 5 cm to 6 cmbased on output of extrusion and speed of take off equipment.

[0136] The resulting core material of the invention has a cross sectionof 42 cm by 5.7 cm; uniform cell size (<0.2 mm); no voids between thestrands; and a foam density of 135 kg/m³. The core material so producedhad the following characteristics: Characteristic Value Density 135kg/m³ Average cell size  0.2 mm Open-cell content <10%

6.6 Example 6

[0137] The procedure of Example 5 above was followed using 0.5 weightpercent nitrogen as blowing agent. The resulting core material of theinvention had a cross section of 40 cm by 3.80 cm; uniform cell size(<0.1 mm); no voids between the strands; and a foam density of 350-400kg/m³.

[0138] The core material so produced had the following characteristics:Characteristic Value Density 350-400 kg/m³ Average cell size <0.1 mmOpen-cell content <10%

6.7 Example 7

[0139] The procedure of Example 1 was followed except that: (1) theshaping process and shaper of Example 4 was used; (2) 0.3 weight percenttalc was included in the foamable gel as a nucleating agent; and (3) 0.9weight percent CO₂ was included in the foamable gel as the blowingagent. The resulting core material of the invention had a cross-sectionof 420 cm×2.3 cm, a uniform cell size (<0.2 mm); no voids between thestrands; and a foam density of 125 kg/m³. The core material so producedhad the following characteristics: Characteristic Value Density 125kg/m³ Average cell size  0.2 mm Open-cell content <10%

6.8 Example 8 Anisotropic Properties of Core Materials of the Invention

[0140] This Example demonstrates the pronounced anisotropic propertiesof core materials of the invention. Samples were selected from a densityrange of about 80 kg/m³ to about 150 kg/m³. The PET core materials ofthe invention were prepared according to Example 1 and had an averagecell size 0.1 mm and an open-cell content <10%. Test samples wereprepared by binding core materials of the invention (7 cm by 3.5 cmplanks) into a block then cutting ½″ panels in the grain directionaccording to the procedure set forth in U.S. Pat. No. 4,536,427 (issuedAug. 20. 1985), hereby incorporated by reference herein. The testsamples were cut from the block as end-strand panels or as flat-strandpanel as appropriate depending on whether the test would be performed byapplying force in the end-strand, transverse, or longitudinaldirections.

[0141] 6.8.1 Shear Tests

[0142] The shear tests were conducted according to the procedure setforth in ASTM C-273 as discussed in Section 5.4.8 above. The results areshown in FIGS. 5-7, which are respectively graphs of: shear strengthversus density (FIG. 5); shear modulus versus density (FIG. 6); andshear elongation at break versus density (FIG. 7). The shear tests wereconducted in the end-strand direction (represented by a “♦” in FIGS.5-7), transverse direction (represented by a “▪”), and longitudinaldirection (represented by a “Δ”) respectively.

[0143]FIG. 5 shows that the shear strength of core materials of theinvention differs depending on the relationship between theapplied-force direction and the strand direction. The most evidentdifference is between shear strength in the end-strand direction versusshear strength in the transverse direction. As shown in FIG. 5, theshear strength difference ranges from about 60% higher in the end-strandover the transverse direction at lower densities to about 130% higher inthe end strand over the transverse direction at higher densities.

[0144]FIG. 6 shows that the shear modulus of core materials of theinvention differs depending on the relationship between theapplied-force direction and the strand direction. The most evidentdifference is between shear modulus in the end-strand direction versusshear modulus in the transverse direction. As shown in FIG. 6, the shearmodulus in the end-strand direction is roughly 100% greater than that inthe transverse direction over the density range.

[0145]FIG. 7 shows that the shear elongation at break point of corematerials of the invention differs depending on the relationship betweenthe applied-force direction and the strand direction. The most evidentdifference is between shear elongation at break point in the end-stranddirection versus shear elongation at break point in the longitudinaldirection. As shown in FIG. 7, the shear-elongation-at-break-point inthe end-strand direction is roughly 170% greater than in thelongitudinal direction over the density range. 6.8.2 Tensile Properties

[0146] The tensile tests were conducted according to the procedure setforth in ASTM C297-94 as discussed in Section 5.4.7 above. The resultsare shown in FIGS. 8-9, which are respectively graphs of tensilestrength versus density (FIG. 8) and tensile modulus versus density(FIG. 9). The tensile tests were conducted in the end-strand direction(represented by a “♦”) and the transverse direction (represented by a“▪”) respectively.

[0147]FIG. 8 shows that the tensile strength of core materials of theinvention differs depending on the relationship between theapplied-force direction and the strand direction. As shown, the tensilestrength in the end-strand direction is roughly 900% greater than thatin the transverse direction over the density range.

[0148]FIG. 9 further shows that the tensile modulus in the end-stranddirection is roughly 1100% greater than that in the transverse directionover the density range.

[0149] 6.8.3 Compression Properties

[0150] The compression tests were conducted according to the procedureset forth in ASTM C365-00 as discussed in Section 5.4.6 above. Theresults are shown in FIGS. 10-11, which are respectively graphs ofcompression strength versus density (FIG. 10) and compression modulusversus density (FIG. 11). The compression tests were conducted in theend-strand direction (represented by a “♦”) and the transverse direction(represented by a “▪”).

[0151]FIG. 10 shows that the compression strength of core materials ofthe invention differs depending on the relationship between theapplied-force direction and the strand direction. As shown, thecompression strength in the end-strand direction is roughly 400% greaterthan that in the transverse direction over the density range.

[0152]FIG. 11 further shows that the compression modulus in theend-strand direction is roughly 50% greater than that in the transversedirection over the density range, with a more pronounced effect athigher densities.

[0153] In sum, this example demonstrates that the core materials of theinvention are significantly anisotropic in character.

6.9 Example 9 Improved Properties of Core Materials of the InventionOver Standard PET Foam Boards Produced by Extruding Foamable PET Througha Conventional, Rectangular Slot Die

[0154] This Example compares the shear properties of PET core materialsof the invention against those of standard PET foam boards produced byextruding foamable PET through a conventional, rectangular slot die(“conventional PET foam boards”). The shear properties of the PET corematerials of the invention were tested in the longitudinal direction.

[0155] The conventional foam boards and core materials of the inventionused in this Example ranged in density from about 80 kg/m³ to about 215kg/m³. Test sample of core materials of the invention were prepared asin Example 8 (6″×2.5″×0.5″), and the conventional PET foam boards (also6″×2.5″×0.5″) can be obtained from M&G Polymers, Patrica, Italy. Boththe conventional foam boards and the core materials of the invention hadan average cell size of about 0.1 mm and an open-cell content of about<10%. The shear tests were conducted according to the procedure setforth in ASTM C-273 as discussed in Section 5.4.8 above.

[0156] The results are shown in FIGS. 12-14, which are respectivelygraphs of shear strength versus density (FIG. 12); shear modulus versusdensity (FIG. 13); and shear elongation at break versus density (FIG.14). The core materials of the invention are represented by “▪” and theconventional foam boards are represented by “♦”.

[0157] These graphs show: (1) that the shear strength of the PET corematerials of the invention are about 50% higher than that ofconventional foam boards over the entire density range; (2) the corematerials of the invention have a higher shear modulus by about 25%-85%over that of the conventional foam boards over the density range; and(3) the shear elongation at break of core materials of the invention isabout 100%-200% higher than that of the conventional foam boards overthe density range. This Example illustrates that the core materials ofthe invention are far superior to conventional foam boards comprisingthe same composition.

[0158] Although the present invention has been described in considerabledetail with reference to certain preferred embodiments and versions,other versions and embodiments are possible. Therefore, the spirit andscope of the appended claims should not be limited to the description ofthe versions and embodiments expressly disclosed herein. The referencesdiscussed in Background Section 2 are not admitted to be prior art withrespect to the invention.

What is claimed is:
 1. A method for making a composite comprising: (a)extruding a foamable gel comprising a blowing agent and a foamablepolyester through a multi-orifice die to give a plurality of strands;(b) foaming the strands to form a multi-stranded foamed article; (c)shaping the multi-stranded foamed article to give a core material; and(d) bonding the core material to one or more structural skins.
 2. Themethod of claim 1, further comprising coalescing the strands afterfoaming the strands.
 3. The method of claim 1, wherein foaming thestrands comprises expanding the blowing agent.
 4. The method of claim 2,wherein coalescing the strands produces inter-strand voids.
 5. Themethod of claim 4, wherein shaping the multi-stranded foamed articleremoves substantially all the inter-strand voids.
 6. The method of claim1, wherein the core material is shaped to produce a plurality ofdiscrete volumes, each discrete volume comprising an interior sectionand a corresponding jacket, wherein the average cell size in theinterior section is larger than the average cell size in the jacket. 7.The method of claim 1, wherein the core material is bonded between twostructural skins.
 8. The method of claim 1, wherein the one or morestructural skins comprises a thermoplastic polymer, a thermosettingpolymer, wood, an inorganic material, or a metallic material.
 9. Themethod of claim 8, wherein the thermoplastic or thermosetting polymercomprises glass fibers, metallic fibers, inorganic fibers, or carbonfibers.
 10. The method of claim 1, wherein the one or more structuralskins comprise a structural sandwich composite.
 11. The method of claim1, wherein bonding the core material to the one or more structural skinscomprises applying a resin to the core material or to the one or morestructural skins.
 12. The method of claim 1, wherein bonding the corematerial to the one or more structural skins comprises applying heat.13. The method of claim 1, wherein bonding the core material to the oneor more structural skins comprises vacuum bagging.
 14. The method ofclaim 1, wherein the foamable gel comprises a nucleating agent, a fireretardant, or a reinforcing agent.
 15. The method of claim 1, whereinthe foamable polyester comprises foamable polyethylene terephthalate,foamable polybutylene terephthalate, foamable polyethylene naphthalate,a foamable copolymer of polyethylene terephthalate, a foamable copolymerof polybutylene terephthalate, a foamable copolymer of polyethylenenaphthalate, or a mixture thereof.
 16. The method of claim 1, whereinthe foamable polyester comprises foamable polyethylene terephthalate.